Modern internal combustion engines must meet stringent emission standards that include limits on the amount of soot, oxides of nitrogen (hereafter “NOx” to include nitric oxide (NO) and nitrogen dioxide (NO2)), carbon monoxide (CO), unburned hydrocarbons (HC), and various greenhouse gases that may be emitted from an engine in operation. Many engines now utilize aftertreatment systems to reduce engine-out emissions to allowable regulatory levels before release to the atmosphere, including lean-burn, compression-ignition engine systems such as diesel engines and stoichiometric, spark-ignition engine systems such as gasoline engines.
The operating conditions of an engine, including the ratio of air to fuel used and the peak combustion temperature within the engine, may affect the amount and type of emissions generated. A “lean mixture” is one in which the ratio of air to fuel is greater than the stoichiometric mixture, which is the ratio where exactly enough oxygen is available to consumed all of the available fuel. For example, gasoline has a stoichiometric air-to-fuel ratio (“AFR”) of about 14.7:1 by mass, meaning the mixture is stoichiometric when about 6.8% of the mass of the mixture is fuel (i.e., gasoline). Lower AFRs, referred to as “rich mixtures,” may result in unreacted fuel in the exhaust gases due to an insufficient amount of oxygen. Unreacted fuel may also be the result of insufficient mixing of the fuel within the charge gas mixture. Higher, lean AFRs may cause the combustion process to become unstable due to an insufficient concentration of fuel to maintain the combustion process or due to a slower combustion reaction that does not run to completion within a single combustion stroke of the engine, both of which may lead to increased HC emissions and lower thermal efficiency. Further, lean AFRs tend to generate a mix of exhaust gases that render the conventional and widely-used three-way catalytic converters ineffective because three-way catalytic converters depend on a specific balance of exhaust species to effectively complete the necessary oxidation and reduction reactions to eliminate undesirable emissions.
Lean-burn systems may expel more NOx emissions than an engine using a stoichiometric AFR due to an excess of oxygen and nitrogen in the combustion process and the difficulty of catalyzing and reducing such emissions. Nonetheless, lean-burn systems may be more fuel efficient due to the lower quantity of fuel added to a given volume of air in the combustion process. Exhaust gas recirculation (“EGR”), in which a portion of the engine's exhaust gas flow is routed into the intake flow into the engine's combustion cylinders (referred herein as “EGR gases”), has been used in both lean-burn and stoichiometric systems to reduce engine-out emissions. However, the use of lean AFR in spark-ignition engine systems has been limited for several reasons, including poor combustion stability, which may increase HC emissions, lower thermal efficiency, and incompatibility with three-way catalytic converter. Accordingly, there remains a need for further contributions in this area of technology.